Suspension Design

[Tsiolkovsky]

thread108-278309

Old one got locked. Its that time again and I got a couple issues.

Can anyone validate my last post showing step by step design process? Is that what you would follow when doing designing suspension?

I also cannot for the life of me see what is compromised when the RC is coincident to CG!! What is affected by doing that?!

 

[NormPeterson]

Compromised? The ability to tune the handling balance with elastic suspension components and damping.

Worst case, both geo RC's are at the sprung CG height, about the only roll you get is from tire deflection, and playing with springs/shocks/sta-bars is unproductive.

In the not-quite-as-bad scenario, it takes relatively bigger changes in the spring, etc., stiffnesses to effect a given handling shift. This has ride quality implications, and ultimately the stiffness of the chassis might prove to set an effective limit on adjustability no matter how stiff you care to make the suspension.


Norm

 

[BrianPetersen]

Roll center coincident with center of gravity implies (for any vehicle of somewhat "normal" shape!) an extremely high roll center.

One bad side effect of doing that, is that if that wheel goes over a one-wheel bump, the geometry causes the tire contact patch to kick sideways. At a minimum, that's going to cause some interesting tire wear. If this is for a front suspension, that's going to cause a big, unpleasant, sideways kick in the steering wheel. It's also going to cause a rough ride ("for every action, there is an equal and opposite reaction" - the sideways kick of the tire contact patch will be translated into a sideways kick of the entire bodywhell, too).

If this is an independent suspension, another bad side effect is that the geometry of ANY high-roll-center independent suspension causes an upward jacking effect in response to cornering loads. For an example of this, look at original (air cooled) VW Beetle, original (first design) Chevrolet Corvair, original (rear engine) Fiat 500 and 126, and a variety of other cars from back then. The common design element is a swing-axle rear suspension design which inherently has a very high roll center. There is a video out there on Youtube of a Fiat 126 cornering, the rear end visibly jacking itself up, and the car basically "tripping" over its own axles and rolling over.

All of these suspension designs are now regarded as obsolete. All modern independent suspensions have much lower roll centers.

You can get away with a higher roll center on a beam-axle suspension, but it still has a bad effect on the ride quality if the roll center is too high. The sideways-kicking over one-wheel bumps is still there.

Compare the ride qualities of a Fox-body Ford Mustang to the current model if you want an example - particularly if the owner has gone ahead and put in stiffer suspension bushings, particularly for the upper diagonal links of the "quadralink" a.k.a. "quadrabind" suspension on the Fox-body. The Fox body uses a pair of diagonal links that place the roll center above the differential housing. The new one uses a panhard rod that is roughly at axle height.

You can make a quadralink axle suspension ride decently (it was commonly used on luxury cars back in the day) ... if you use really soft, compliant bushings and tires with soft sidewalls, to compliantly absorb some of the side kick. Handling suffers accordingly.

 

[Tsiolkovsky]

Apart from high tyre scrub, high jacking forces and the inability to tune handling balance, are there any other downsides?

 

[NormPeterson]

Little or no roll = one less piece of feedback to the driver about what is going on.

A little roll can be used to discourage drivers from driving their vehicles too close to the limits of cornering grip/handling behavior/overturning.


Norm

 

[GregLocock]

Against that, experiments with active suspension cars indicate that the increased stability of the driver's head in a zero roll car is very beneficial for laptimes. But you do need to detect latacc in a different way, or drive using more steering feel (I don't know what they did in that experiment).



Cheers

Greg Locock


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[Tsiolkovsky]

If those are the downsides off the top of your heads can anyone explain what Milliken means by this paragraph because ive been trying to understand it for ages!

The higher the roll center the smaller the rolling moment about the roll center (which must be resisted about the springs); the lower the roll center the larger the rolling moment. You will also notice that with higher roll centers the lateral force acting at the roll center is higher off the ground. This lateral force X the distance to the ground can be called the non rolling overturning moment. So roll center heights are trading off the relative effects of the rolling and non rolling moments.

 

[NormPeterson]

What it means is that the overall overturning moment for a given vehicle at a given latacc remains constant (assuming that the CG height does not change).

All you're doing by moving the roll centers vertically is shifting how much of that overturning moment is being resisted by the suspension geometry (non-rolling) and how much is taken through the elastic elements (springs/bars and dynamically the dampers).

You don't gain or lose total lateral load transfer by relocating what's usually a virtual point anyway, but what it comes back to is how much of this load transfer you can shift around for handling tuning purposes.


Norm

 

[Tsiolkovsky]

It seems as though in your last sentence you are saying changing RC does not affect the loads on all four tyres but then say you can shift around load transfer by doing so.

Could you clarify this?

From what physics and intuition tells me, playing around with how much the load is on the springs i.e. changing RC, does NOT affect loading on all fours.

 

[NormPeterson]

You can't change how much. Only where it goes.


Norm

 

[BrianPetersen]

Picture your hypothetical car in which the roll center is at CG height front and rear. Although the car does not lean in corners, suppose you have an oversteering or understeering condition that you would like to tweak. You are screwed.

Now take that car and put the roll centers at a more sensible (lower) height and have antiroll bars take up the portion of the corner load transfer that the geometry does not take care of. The car leans in corners now, but suppose you have an oversteering or understeering condition that you would like to tweak. Simple - change the diameter of the front or rear antiroll bars as necessary, or change the mechanical advantage that the wheel motions have on the antiroll bar linkage. No problem.

DO remember that independent suspensions with very high roll centers inherently have a large jacking component to imposed cornering loads, which raises up the car (and the C of G height) during cornering. In extreme cases, it can cause a rollover.

 

[GregLocock]

...or perhaps more directly, the total load transfer in that often quoted but rarely seen state of constant latacc, the total load transfer is given by the schoolboy equation, how it is distributed between the two axles is more complex.



Cheers

Greg Locock


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[Tsiolkovsky]

Hello all.

Thanks for the input, I understand a bit more now. Frustrating that the vehicle dynamics books couldn't put it across more simply.

Anyway Ive hit a real dead end here. I do not plan to design the steering knuckle that would be too much work. I plan to take it from existing small cars/kit cars. There are problems with this. Using existing knuckles means using compliant ball joints (with different stud lengths etc...), bearings etc... And it is IMPOSSIBLE to get dimensions on the web for ANY knuckle/ball joint manufacturer distributor. No catalolgues/specs, nothing. Only a pretty picture and code number (oh and what car it can be used on). This is frustrating. Now you may be asking why im worrying about detail design when I haven't even designed for my RC, camber gain, nothing! Well, answer is, if I design for dimensions to be X,Y and Z for knuckle height (for example) and it doesn't come in that size configuration, it'll be wasted work.

I would really really appreciate any detail catalogues dimensioning existing knuckles, ball joints, heck even hubs, although thats easy to manufacture.

 

[patprimmer]

I would be inclined to just pick a very popular madel that is about the correct weight to have suitable sized ball joints and then buy a few and measure them then build the arms and position mounting points to match.

Regards
Pat
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[WolfHR]

If I understand correcty (I think the most descriptive explanation was in Carrol Smith's 'Tune to win')- higher jacking force also means less centripetal force can be generated with same lateral forces on tyres. I have tried to dissuade students at my Uni from using such concept on their car and my rather crude calculations showed they were to lose (even with highly exaggerated tyre load sensitivity) about 7-8% of centripetal force (and that should be approx. 4% of the cornering speed).

 

[patprimmer]

In my previous post, madel should obviously be model. I think I changed my mind about the sentence structure half way through a word. Anyway fat fingers doesn't this time as a and o are at opposite ends of the keyboard.

Regards
Pat
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